1. Field of the Invention
The present invention relates to integrated circuit structures and to methods of their fabrication and, in particular, to integrated circuit fabrication techniques that prevent copper diffusion and permit the use of materials having a low dielectric constant (k) in copper interconnect structures.
2. Discussion of the Related Art
There are a number of issues associated with the utilization of copper interconnects in high density integrated circuits. For example, copper has a high diffusivity in oxide and silicon, even at room temperature. If copper diffuses from the interconnect wiring into the underlying active electrical devices, then these devices can fail to operate. Therefore, suitable confinement of the copper in the interconnect wires and thus, protection of the electrical devices is imperative.
The standard industry approach for the utilization of copper interconnects is to use barrier metals such as titanium nitride (TiN), tantalum (Ta), tantalum nitride (TaN) and/or tungsten nitride (WN) to prevent copper diffusion from the wires. However, this is a challenging task because barrier metal deposition processes must provide conformal coverage of the dual damascene structures commonly used in today's device structures. Moreover, the diffusion properties of the barrier metal in high aspect ratio dual damascene structures must meet high performance criteria.
Also, the semiconductor integrated circuit industry is leaning toward integration of copper with low k dielectrics. This creates a new set of problems. Low k materials are unstable at high temperatures. It is difficult to etch dual damascene structures in low k materials. Furthermore, selectivity of copper chemical mechanical polishing (CMP) to low k materials is poor.
Commonly-assigned U.S. patent application Ser. No. 09/295,898, filed Apr. 21, 1999, by Vassili Kitch and Michael E. Thomas, titled "Self-Aligned Interconnect Using High Selectivity Metal Pillars and a Via Exclusion Mask", discloses a process for forming a via in a semiconductor device that uses a self-aligned metal pillar to connect conductive interconnect layers separated by a dielectric. The upper surface of the pillar comprises a conductive cap material, e.g., tungsten, that exhibits high selectivity, i.e. greater than 5:1, to the etch of the overlying metal interconnect structure. Since the overlying metal formed on the conductive cap is typically aluminum or another material having high selectivity to tungsten, etching of the conductive layer during its patterning will not etch the underlying tungsten cap in the event of misalignment. Application Ser. No. 09/295,898 is hereby incorporated by reference to provide additional background information regarding the present invention.
Commonly-assigned U.S. patent application Ser. No. 09/295,838, filed Apr. 21, 1999, by Vassili Kitch, titled "Self-Aligned Copper Interconnect Structure and Method of Manufacturing Same", discloses a process for forming a copper-based interconnect structure using via pillars. In accordance with one embodiment of the disclosed invention, a lower interconnect layer includes a conductive diffusion barrier layer, a first copper layer formed on the diffusion barrier layer and a conductive etch stop layer formed on the first copper layer. An upper layer of conductive material, e.g., copper or tungsten, is then formed on the etch stop layer. The metal stack is then etched. If the upper conductive layer is copper, then a conventional hardmask, e.g., thick silicon dioxide (SiO2), is used to etch the upper copper layer, the etch stop layer, the first copper layer and the lower diffusion barrier layer to define the patterned metal stack. If the upper conductive layer is tungsten, then the tungsten itself can serve as a hardmask. In this case, patterned photoresist is used to etch through the tungsten; then the remainder of the stack is etched using the patterned tungsten as the hardmask. Following definition of the metal stack, a thin layer of dielectric material, e.g., silicon oxynitride (SiON) or silicon nitrite (Si.sub.3 N.sub.4), having good barrier properties against copper diffusion, is then deposited as a coating over the patterned metal stack. First dielectric material is then deposited, filling the gaps between the coated metal stack. The structure is then planarized using chemical mechanical polishing (CMP), leaving the upper surface of the upper conductive layer exposed. The exposed surface of the upper conductive layer is then masked and etched to define conductive via pillars. The etch stop layer of the lower interconnect layer serves as an etch stop to the via pillar etch. The sidewall surfaces of the via pillars that were exposed during the via pillar etch are then again coated with the barrier dielectric material and the gaps between the coated via pillars are filled with dielectric material. The dielectric material is then planarized to expose the upper surface of the via pillars. An upper conductive layer, preferably copper-based, is then formed in electrical contact with the exposed upper surfaces of the via pillars. This completes the fabrication of two copper-based interconnect layers with conductive via pillars formed between the two interconnect layers. Application Ser. No. 09/295,838 is hereby incorporated by reference to provide additional background information regarding the present invention.
Commonly-assigned U.S. patent application Ser. No. 09/295,892, filed Apr. 21, 1999, by Kevin C. Brown, titled "Self-Aligned Copper Interconnect Architecture With Enhanced Copper Diffusion Barrier", discloses a process that provides self-aligned vias in a copper-based multi-level interconnect structure using a conformal, metal-like material, such as CVD titanium nitride or PVD tantalum nitride, as a copper diffusion barrier. In accordance with one embodiment of the disclosed invention, a lower conductive interconnect layer includes a diffusion barrier layer, a first copper-based layer formed on the diffusion barrier layer and an etch stop layer formed on the copper-based layer. A second copper-based layer is formed on the etch stop layer and an optional conductive antireflective diffusion barrier layer is formed on the second copper-based layer. The upper antireflective diffusion barrier layer is then patterned and etched and utilized as a conventional hardmask to etch the second copper-based layer, the etch stop layer, the first copper-based layer and the lower diffusion barrier layer to define the patterned metal stack. A thin layer of conformal metal-like material, e.g., CVD TiN or PVD TaN, having good barrier properties against copper diffusion is then deposited over the patterned stack. An isotropic spacer etched-back of the conformal conductive barrier material is then performed such that barrier material is removed from the horizontal surfaces, leaving conductive diffusion barrier material on vertical sidewalls of the patterned stack. First dielectric material is then deposited, filling the gaps between the patterned stack. The structure is then planarized, e.g., using chemical mechanical polishing (CMP), leaving the upper surface of the patterned antireflective diffusion barrier layer exposed. The exposed antireflective diffusion barrier layer is then masked and etched to define copper-based via pillars that have caps formed with a conductive antireflective material. The etch stop layer serves as an etch stop to the copper etch. If the copper via pillar etch is selective to the conductive barrier material, then, to avoid shorts between via pillars, the via pillar etch must be followed by an isotropic etch of the exposed conductive barrier material that is selective to copper. If the copper via pillar etch is not selective to the conductive barrier material, then exposed conductive barrier material will be removed by the copper etch and the process can proceed. Since the conductive diffusion barrier layer has now been removed wherever it was exposed by the copper etch, and since the exposed sidewalls of the via pillars are thus not now protected by a diffusion barrier, a layer of conformal diffusion barrier dielectric material, e.g., silicon nitride, is deposited over exposed surfaces. Following deposition of the conformal diffusion barrier dielectric material, gaps between the via pillars are filled with dielectric material. The dielectric material is then planaraized to expose the upper surface of the antireflective diffusion barrier material that caps the via pillars. An upper conductive layer is then formed in electrical contact with the exposed pillar caps. The upper conductive layer comprises a material having high selectivity to the material of the antireflective diffusion barrier layer. Therefore, etching of the upper conductive layer during its patterning will not etch the underlying via pillars in the event of mask misalignment. If an antireflective layer is not utilized on the copper via pillar layer, then a conventional hardmask, e.g. thick silicon dioxide, is used to pattern the metal stack. A hardmask is needed because a conventional photoresist mask will not survive the high temperatures needed to etch copper. Use of a conformal metal-like diffusion barrier material, such as CVD titanium nitride or PVD tantalum nitride, improves over use of other copper barrier diffusion materials, such as silicon oxynitride. Application Ser. No. 09/295,892 is hereby incorporated by reference to provide additional background information regarding the present invention.